The invention arose during continuing development efforts in two cycle outboard marine engines.
The invention particularly addresses and solves problems of thermal distortion of the piston relative to the cylinder bore. In the present invention, the shape of the piston is matched to the cylinder bore to accommodate thermal distortion.
The invention also reduces the tendency of piston scuffing which is caused by a breakdown of lubrication between the piston skirt and the cylinder bore sidewall. This can cause local melting of metal material which then smears along the side of the cylinder bore sidewall and causes piston sticking.
In conventional designs, the piston shape is cammed across the wrist pin, i.e. the piston has a cross section therethrough along a plane perpendicular to the direction of axial movement of the piston, which cross section has an outer peripheral generally oval shape with a minor axis generally along the smallest diameter dimension, which minor axis extends along the wrist pin.
In the present invention, the minor axis is offset from the wrist pin toward a line through the distally opposite intake and exhaust ports, and preferably the minor axis extends through such ports. It was found that offsetting the minor axis of the oval from the wrist pin toward a line through the intake and exhaust ports, and preferably through such ports, provided superior performance and piston profile tolerance to the cylinder bore. It is believed that this performance is due to such minor axis enabling cylinder bore expansion at the ports toward the piston. This is believed particularly significant at the exhaust port, where higher temperatures occur.
The invention also enables much tighter tolerances between the piston skirt and the cylinder bore sidewall, which in turn provides improved sealing in a two stroke engine. In addition to reduction of piston scuffing and tighter skirt to bore clearances, the invention also provides improved low speed running quality and a reduction in piston slap noise.
The invention further enables piston profile shaping to match cylinder bore distortion, which latter has been found difficult to quantify. In the present invention, the thrust load is evenly distributed to create an oil film between the cylinder bore sidewall and the piston to ensure continued hydrodynamic lubrication and resistance to scuffing. This is done by identifying the degree of cylinder bore distortion and in which cylinder such distortion occurs, and then profiling the piston to match same under conditions of thermal distortion, i.e. once bore distortion and piston expansion are known, the shapes to be machined when cold can be accomplished to render relative shapes to support hydrodynamic lubrication .